In case this isn't known: Create a guide. Lock it. Then go back into the guide and change the position (x/y). Then go to the undo history, and undo that.

The guide doesn't move when undone. But, if you save the drawing, close and re-open, the guide is back to where it should be. Essentially: the undo is working, but the guide isn't being redisplayed to reflect that.

This appears to be due to the guide being locked. If I unlock it, move it, undo, then the guide moves back to where it was. It seems like the undo feature is respecting the lock in a way it shouldn't (basically: if there's any guide-related undo history, it should be acted upon. If it was allowed to happen while locked, it should be allowed to be undone.).